mapSource: Get tile server information from service.json

Attempt to download a service.json file from gnome.org. The
file will define all information needed to create a libchamplain
source. This will help us to change the tile-server without needing
to update the application.

If the call to gnome.org fails. Fallback to a default service.json.

https://bugzilla.gnome.org/show_bug.cgi?id=769352